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Hi,
Yes this is possible (in fact I just did it yesterday to a new 2011 13" pro). You have to replace the dvd superdrive with a hdd enclosure that fits in its place, it goes by several names (optibay,data doubler, etc). For my 13" mbp I bought a 9mm enclosure off of ebay for $15 and it works fine.

My current config is 120GB Agility 3 ssd in hdd bay and 320GB stock mbp hdd in optibay.
 
I did the same as above, but went 240GB Vertex 3 in the HDD bay and 1TB Samsung drive in the optibay, used a $15 optibay from ebay, 100% as good as the $75 ones.

The SSD is in the HDD bay because its the only SATA3 port, the optibay is SATA2 (this is in my MBP, yours may have dual SATA3).
 
There are two things you should consider:

1. The shock sensor will not work for the HDD in optibay slot. Hence, if you use a non-SSD drive it won't be protected from bumps.

2. Optibay has SATA2 (3Gb/s) instead of SATA3 (6Gb/s). If you decide to use SSD with optibay slot it might not be worth buying the fastest SSD money can get (there are one or two out there that can saturate SATA3) but instead get a speedy mid-range SSD.

Also, one more point to note:

256GB SSDs are *faster* than 128GB SSDs (at least the current gen). This is simply because 256GB consists of more physical memory chips and hence there are more channels than can be written to/read from at the same time. If 128GB was made of 4 memory chips, 256GB would be made of 8. There is a big speed gain there; so I guess it should be better to get a decent 256GB SSD instead of most expensive 128GB.
 
There are two things you should consider:

1. The shock sensor will not work for the HDD in optibay slot. Hence, if you use a non-SSD drive it won't be protected from bumps.

2. Optibay has SATA2 (3Gb/s) instead of SATA3 (6Gb/s). If you decide to use SSD with optibay slot it might not be worth buying the fastest SSD money can get (there are one or two out there that can saturate SATA3) but instead get a speedy mid-range SSD.

Also, one more point to note:

256GB SSDs are *faster* than 128GB SSDs (at least the current gen). This is simply because 256GB consists of more physical memory chips and hence there are more channels than can be written to/read from at the same time. If 128GB was made of 4 memory chips, 256GB would be made of 8. There is a big speed gain there; so I guess it should be better to get a decent 256GB SSD instead of most expensive 128GB.

To rebut the shock sensor issue, hard drives have existed for decades before shock sensors as have laptops, modern hard drives have their own shock protection across almost all brands. Its more of a marketing angle than an actual feature. How many people do you guys know that have had a hard drive fail due to a shock that didn't completely destroy the rest of the laptop.
 
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